1. Field of the Invention
The present invention relates generally to the field of light polarization. More particularly, and not by way of limitation, the present invention relates to the polarization of light propagating in an optical fiber.
2. Description of the Prior Art
In many applications, it is important to obtain a pure state of light polarization while, at the same time, keeping the light guided inside the core of an optical fiber. In one important application, for example, there is a need to adjust the polarization of light prior to sending the light to an external electo-optical modulator.
One known approach to providing linearly polarized light in an optical fiber is to utilize a special optical fiber; for example, a special polarizing fiber manufactured by 3M(copyright). In such a fiber, one state of polarization experiences a very high loss while the other state suffers relatively low losses. According to the manufacturer, a straight 3 meter length of this special fiber is capable of obtaining a 20 dB extinction ratio, while coiling the same length in a 3 cm diameter rod provides an extinction ratio of over 40 dB.
Other approaches for providing an in-line polarizer employ technology utilized to fabricate polished fiber couplers or D-shaped fibers. In these alternative approaches, the flat part of the fiber is coated with different layers which include a buffer layer and a metallic absorbing layer.
Each of the above-described approaches is not fully satisfactory. For example, the approach utilizing the special 3M(copyright) polarizing fiber implies the use of several meters of a fiber that is rather expensive. Furthermore, in the telecommunications field, a problem that is common to approaches based on the use of special fibers is the coupling efficiency of light from a standard telecomm fiber to the polarizing fiber. The approaches that utilize technologies to fabricate polished fiber couplers or D-shaped fibers suffer from the problem that very well-controlled film deposition and cladding thickness are required.
An optical fiber that incorporates a grating to polarize light propagating therethrough is also known in the art. U.S. Pat. No. 5,546,481 to Meltz et al., for example, discloses a single polarization fiber/amplifier that includes a nonpolarization preserving fiber having a grating tap incorporated therein. The grating tap in the Meltz et al. patent is described as being oriented at a predetermined angle, as having a predetermined grating spacing and grating strength, and as having a grating length that extends substantially the entire length of the optical fiber so as to couple-out of the optical fiber a predetermined amount of one polarization over a predetermined wavelength range while passing the second polarization as an output light from the optical fiber.
Although Meltz et al. provides linearly polarized light guided in a normal optical fiber, rather than a special fiber as described above, polarization in Meltz et al. is attained by Brewster angle reflection. As a result, in Meltz et al., there is a disadvantage in that light in the rejected polarization state is coupled out of the fiber and cannot be used.
The present invention provides a method and apparatus for polarizing light propagating in an optical fiber that does not require the use of a special fiber and that provides several advantageous properties generally not found in known optical fiber polarizers.
An apparatus for polarizing light propagating in an optical fiber according to the present invention comprises an optical fiber having a Fiber Bragg Grating (FBG) section of predetermined length, and a force applying mechanism for applying a lateral force to a predetermined portion of the Fiber Bragg Grating section.
It has been discovered that by providing an optical fiber with a Fiber Bragg Grating section, separation of the two polarization states of light propagating in the optical fiber can be obtained by applying a lateral force to a predetermined portion of the Fiber Bragg Grating section. As a result, polarized light can be obtained using a standard telecomm fiber or other optical fiber in which a strong refractive index modulation can be obtained, without requiring the use of more costly special fibers.
According to an embodiment of the present invention, the light polarization can be tuned by varying the position where the force is applied by the force applying mechanism, or by controlling the amount of the force applied. In particular, if the pressed portion of the FBG is a small portion of the total grating length, e.g., about one percent of the grating length, it is possible to have an intraband spectral hole in the reflectance spectrum for one polarization, with no destructive interference for the other polarization. In this case, the small length is not long enough to present significant reflectivity by itself such that the applied force introduces a phase difference between two long parts of the grating separated by the small pressed region. On the other hand, if the pressed portion of the FGB is equal to or greater than about 10 percent of the grating length, a main loss peak and a secondary loss peak can be observed that correspond to the fast and slow axes of the pressed region. In this case, the pressed region has to be sufficient to present a strong reflectivity by itself; i.e., the pressed region will act as an asymmetric grating. In one embodiment, for example, a uniform grating can be provided and its total length pressed. Thus, by pressing the FBG in a controlled manner on a predetermined portion of the grating length, polarization of light propagating in the optical fiber can be readily achieved.
In the present invention, polarized light is attained by applying force to a portion of an FBG section of an optical fiber to introduce a difference in Bragg wavelength of each polarization state. As a result, light in the rejected polarization state is guided backwards in the fiber rather than being coupled out of the fiber. Accordingly, with the present invention, it becomes possible to utilize the light in the rejected polarization state, if desired. Furthermore, the apparatus for polarizing light according to the present invention operates at two spectral bands with reversed effects for each polarization state. In other words, with the present invention, in a first operating band, polarization state Y will be transmitted and polarization state X will be reflected; while in a second operating band, the effect will be the opposite, where the xy plane is perpendicular to the propagation axis z of the fiber, and x is defined as the direction along the applied force.
According to a further embodiment of the invention, inasmuch as the polarization is dependent on the force applied to the FBG portion, the polarizing apparatus can be easily switched on or off simply by applying or releasing the force.
According to other embodiments of the invention, the FBG can be homogeneous or chirped. When the FBG is homogeneous, the operating range is a few nanometers and the center wavelength has some limited tunability. When the FBG is chirped, the polarizing effect takes place within a narrow wavelength window but it can be continuously tuned along the chirped grating reflection spectrum.
According to yet another embodiment of the present invention, the optical fiber can comprise highly birefringent fiber. Although the coupling efficiency is reduced somewhat when using such a fiber, the permanent birefringence allows the apparatus to be used with zero applied force or with less force compared to that required with standard telecomm fiber.
With a light polarizing apparatus according to the present invention, it is possible to use a standard optical fiber rather than a special optical fiber. The optical fiber can be a standard telecomm fiber or another optical fiber in which a strong refractive index modulation can be created. The apparatus is also suitable for mass production procedures.
Yet additional objects, features and advantages of the present invention will become apparent hereinafter in conjunction with the following detailed description of presently preferred embodiments thereof.